What I believe the GP was getting at is that "I could care less" is a corruption of the original "I couldn't care less", and one that makes next to no sense as an expression of contempt.

The sarcastic interpretation makes even less sense. Is the sarcastic interpretation 'I could care more' or 'I could not care less'? At any rate, it is definitely not interpreted as sarcasm today, nor is it spoken with any kind of inflection to denote sarcasm.

It's just a pointless statement that has been echoed enough times that we've become accustom to the sound it makes, much like a parrot that mimics 'Polly wants a cracker'.